1. Field of the Invention
This invention relates to a method of draw forming a desired contour along outer diameter of a tubular workpiece, and more particularly, to such a method for controlling and minimizing an unwanted increase to the wall thickness along the altered contours of the tubular workpiece useful particularly to form a metal ball bat or similarly formed tubular metal workpiece having contours to meeting stringent shape and wall thickness requirements.
2. The Prior Art
While not so limited, the present invention is particularly useful to provide a workpiece for the manufacture of metal ball bats although equally useful to provide a workpiece for the manufacture of contoured components of structural assemblies such as bicycles frames of the type needed for light weight and high strength sports bicycles. The methods of manufacturing ball bats and improvements in the design and materials have been the subject of numerous patents over the past 100 years, most directed to ball bats used in games of baseball and softball. The baseball bat was initially made of hardwood and, to this day, ball bats used in professional baseball leagues are exclusively made of hard woods. Over the years, there has been a great increase in the number of ball bats to meet the demand for the vastly increasing popularly of the sport particularly semi professional, college, and little league baseball and softball organized leagues. There has been a search for better materials to make the ball bat particularly due to a shortage of appropriate hard wood to make baseball bats and the relatively short vulnerable life of the wooden bat. An early approach was to select metal as an alternative material for the hard wood material. U.S. Pat. No. 1,611,858 issued in 1926 in the name of L. Middlekauff discloses a ball bat made from a tapered steel tube, which can be formed by a rolled tapered sheet with mating edges joined along a seam line to form the tube. U.S. Pat. No. 2,340,156 issued in 1944 in the name of T. Taylor discloses a cast construction for a ball bat with thin sidewalls reinforced by ribs longitudinally throughout the length of the bat.
Thereafter, the patent art concentrated on seamless lightweight metal tubing such as aluminum, titanium as the usual and most suitable as a starting material. However, after further experimentation and investigations into various methods of producing ball bats from lightweight metal tubing, one overall requirement existed, namely, the metal bat must closely resemble the operating characteristics of a wood bat. Specifically, the metal bat must exhibit the weight distribution; the feel and sound of the wood bat when hitting a ball. In the final forming of the metal tube into the characteristic outside contour of the ball bat, a rotary swaging process was commonly used for the required metal working operations. As an alternative, U.S. Pat. No. 5,626,050 issued in the name of T. Ploughe discloses the characteristic contour for a metal bat can also be produced by the cold pilger process. The forming operations to achieve the characteristic bat contour by the rotary swaging process or cold pilger process both involved reducing of the outside diameter of the metal tube by applying radially inward compressive forces along the selected length of the metal tube. These radially applied compressive forces also operate to thicken the wall section of the reduced outside diameters of the tube generally in direct proportion to the diameter decrease. U.S. Pat. No. 3,479,030 issued in the name of A. Merola describes that this wall thickening adversely affects the balance and weight distribution along the elongation to the length of the ball bat as compared to a wood bat. U.S. Pat. No. 3,854,316 issued in the name of R. Wilson and the cold pilger process disclosed in U.S. Pat. No. 5,626,050 issued in the name of T. Ploughe disclose counteracting the wall thickening accompanying the diameter reduction by inserting contoured mandrels in the cavity of the tube during the rotary hammer swaging operation and cold pilger process, respectively. The use of the internal mandrel is useful to control the tube wall thickening but significantly added to the metal working costs and greatly increased the stress in the machinery used to reduce the outside diameter of the tube.
The starting of the metal bat forming operation with a variable wall tube blank was proposed as a measure to counteract the wall thickening of the metal tube along the tube length where the reduction to the outside wall diameter occurred. The required variable wall thickness and its location along the metal tube blank so as to result in the desired wall thickness after outside contour forming has been proposed to be accomplished in several ways. U.S. Pat. No. 3,841,130 issued in the name of I. Scott disclosed machining the outside of the metal tubular blank prior to forming of the tube. U.S. Pat. No. 3,807,213 issued in the name of Willis et al and U.S. Pat. No. 4,089,199 issued in the name of F. Siemonsen disclose cold forming a tube blank over a contoured mandrel for producing the desired results of a metal blank with a constant outside diameter and variable wall thickness blank. U.S. Pat. No. 2,240,456 in the name of F. Damer discloses the use of a tapered manipulating mandrel for drawing multiple lengths, constant outside diameter blanks with varying wall thicknesses. Starting blanks with variable wall thicknesses generally solve the problem of controlling the wall thickness and the location of changing wall thickness along the length of the finished ball bat. However, the metal working operation to produce the final outside contour was usually a rotary swaging process which is slow and labor intensive. The radial compressive forces generated by the rotary swaging process create a hardening of the metal along the reduced outside diameter section of the bat especially in the handle area. In the case of aluminum alloys chosen for their hardness for higher performance more costly bats, the hardening of the aluminum metal was so great that the rotary swaging operation to form the required outside contour of the bat required two swaging passes separated by an intermediate annealing operation to insure that the metal did not crack or break during the rotary swaging process.
The present invention seeks to avoid the disadvantages arising out of the use of the rotary swaging and cold pilger processes by the use of drawing a blank only partly through a contoured die or a succession of contour dies. The drawing process operates to reduce the diameter of the metal tubular blank which greatly reduces increases to the tubular walls undergoing the reduction to the diameter particularly as compared to the unwanted thickness increases to the tubular walls when acted on by the rotary swaging or the cold pilger processes. An additional advantage arising out of the use of the contoured dies for draw forming a starting tubular blank is the fact that the drawing operations produce considerably less work hardening of the metal forming the tubular blank. Because of the presences of tension forces for the size reduction operation to such a beneficial extent that the outside diameter of a desired finished ball bat contour can be produced, in most cases, without the need for an intermediate annealing operation even when the tubular metal blank is comprised of alloys exhibiting harder physical properties of aluminum. Thus, the present invention provides the advantage that intermediate annealing can be usually eliminated. The drawing method of the present invention can produce an acceptable product using a constant wall thickness starting blank especially for the lower performance bats. When a thinner tube wall is required in the handle and transition sections of a ball bat than can be produced from a constant wall starting tubular blank, the thinner wall thicknesses can be achieved by starting with a variable wall thinner metal starting blank. The requirement for thinner tube wall exists for the higher performance, more costly ball bats.
It is an object of the present invention to provide a process for reducing the diameter of essentially only a select length of a tubular metal blank by the use of tension applied to pull the metal blank in a die or a succession of dies so that the reduction to the diameter of the tube is accompanied by a minimum of thickening to the wall thickness and which can be enhanced by the selection of a starting blank with a select tapering wall thickness.
It is a further object of the present invention to provide a draw forming operation for a tubular metal blank wherein the succession of drawing operations is used to produce a desired contour without the need for intermediate annealing particularly with workpieces comprised of aluminum or aluminum alloys.
It is still another object of the present invention to provide a drawing operation to use contour dies to produce a product suitable for forming a ball bat high performance characteristics arising out of the formation of thin wall sections enhanced by the selection of a starting blank having a variable thickness wall.
It is another object of the present invention to provide a draw forming operation for a ball bat or similar workpiece wherein multiple drawing operations are performed with the second and any additional drawing operation being controlled by limiting the tension forces applied to the metal blank with such forces being resisted by the contour of the die used for the drawing operation.
In accordance with the present invention there is provided a method of making a contoured outer diameter of a metal ball bat, the method including the steps of; selecting a starting tubular metal blank, and reducing an outer diameter of essentially only a part of the starting tubular metal blank by drawing the metal blank only partly through a contoured die or only partly through each one of a succession of contoured dies.